1. Field of the Invention
The present invention relates generally to gallium arsenide solar cells, and more particularly to forming pure silver ohmic contacts to both n-type gallium arsenide semiconductor materials with substantialy low doping carrier concentrations and p-type gallium arsenide materials.
2. Description of the Prior Art
It is important for gallium arsenide solar cells which generate high current densities to have good ohmic contacts for efficient and reliable operation. The ohmic contact at a metal-semiconductor junction may be defined as one which exhibits linear current-voltage characteristics. A crucial property of the ohmic contact is its specific contact resistivity, that is electrical resistance between the contact and the semiconductor multiplied by the contact area. The specific resistivity of a good ohmic contact should be less than about 10.sup.-3 .OMEGA.cm.sup.2. The importance of good ohmic contacts becomes more significant when it is realized that to collect current within semiconductor solar cells, electrical connections must be made at the metal-semiconductor junctions, and that to maximize junction current flow, it is essential to use the lowest resistance contacts possible.
The formation of ohmic contacts for gallium arsenide cells which exhibit acceptable low resistance depends on many factors. Heretofore, one primary such factor was the use of a highly doped semiconductor at the interface under the contacting metal. With regard to this factor, it will be noted that the present invention contemplates forming pure silver ohmic contacts to the n-type gallium arsenide materials with fairly low doping densities, as will be more fully explained hereinafter. Moreover, a number of other factors such as surface preparation, metal deposition conditions, reproducibility, cost-effective contacting techniques, and satisfactory electrical characteristics must also be considered in the formation of superior quality ohmic contacts.
The requirements for selecting specific contacting metals for making ohmic contacts to gallium arsenide solar cells depends on many of the factors just previously mentioned. Generally, the most widely selected contacting metals are gold-base alloys. Gold alloys are frequently selected because they typically advantageously provide relatively good performing contacts with acceptable contact resistance.
However, in spite of this significant advantage, there are some major problems associated with gold-alloy contact systems, and these problems generally involve cost. For instance, the cost of virtually every step involved in the manufacture of gallium arsenide cells presently substantially exceeds the cost of the manufacturing steps of silicon solar cell counterparts by a large factor. To illustrate further, the alloying process step of gold contact systems normally contributes to the high manufacturing cost because the gold alloys often comprise complex multiple element systems. Consequently, they are fairly expensive to produce.
Similarly, the doping process step of gold contact systems also frequently contributes to the excessive manufacturing cost of this system. For example, as previously mentioned, to achieve ohmicity for the majority of gold-alloy contacts, the metal-semiconductor interface must necessarily be highly doped. Unfortunately, doping must necessarily be carefully performed since a heavy diffusion of donor or acceptor impurities can result in a deterioration of the underlying junction, which can eventually degrade solar performance. Consequently, this step is often costly, as well as time consuming.
Incidentally, it is to be noted that the high doping carrier concentrations at the semiconductor interface may also have a material effect in electrically degrading cell performance. This is so primarily because the lifetime and diffusion lengths of the minority carriers are appreciably decreased as the carrier concentrations are increased. The result, of course, is a reduction in current collection efficiency. In addition to the above production problems, it is also fairly expensive to manufacture gold-alloy contacts at high volumes.
Another major problem associated with some gold-alloy contact systems is aging. For instance, gold-alloy contacts which are made directly to the n-type surfaces are often subject to aging effects as a consequence of damage introduced into the n-type gallium arsenide materials by the alloying process. The effect of aging is generally to degrade the performance of the gallium arsenide cell. It also normally shortens the mean time of failure of the operational cell. Moreover, these conditions usually combine to adversely affect cell stability.
Still another problem is that the pure gold alloys generally possess poor wettability (nonwetting). Poor wetting causes the liquid gold alloys upon heating to stand up in the form of drops at the semiconductor interface instead of spreading, which gives rise to a high specific contact resistivity.
To cope with the aforesaid problems, particularly that of nonwetting, a layer of nickel is deposited over some gold-alloy contacts, such as gold-germanium, to suppress the balling-up effect. Unfortunately, nickel, despite its usefulness in enhancing wetting, is a fast diffuser in gallium arsenide materials, and therefore excessive amounts degrade the gold alloy contact performance.
Another approach to overcome the problems associated with gold-alloy contacts is to replace them with less expensive metal alloys. To this end, silver base alloys are commonly used as an alternative to gold-alloys essentially because they provide quality ohmicity to both n-type and p-type cells at fairly high doping carrier concentrations. Unfortunately, in virtually all silver-alloy contact systems, the complexity of the metallization process and the high cost associated with manufacturing them still remain a severe problem. Additionally, some silver-alloy contacts, such as tin-silver, tarnish when exposed to air. This problem is compounded when the contact is to be bonded by thermal compression to a heat sink.
In a similar approach, a number of pure metals have also been considered as alternative contact systems for gallium arsenide cells. Some of the most widely used pure metal contacts are molybdenum, chromium, titanium, tin, indium, gold, and silver. These metals are attractive because they usually form good performing ohmic contacts to either the p-or n-type materials. Unfortunately, most pure metal contact systems require substantially high levels of doping carrier concentrations to achieve ohmicity, and thus fail to satisfactorily solve the aforesaid associated problems concerning the reduction in current collection efficiency and the adverse effects of excessive diffusion of donors and acceptors during the doping process.
In this regard, it is reiterated that the present invention contemplates using pure silver to form ohmic contacts to the n-type gallium arsenide materials at substantially low density carrier concentrations, as well as to the p-type gallium arsenide materials. Heretofore, it has been well established that for superior quality ohmic contacts to be achieved for the n-type materials, a highly doped semiconductor interface was absolutely necessary. To this end, all known prior art teachings indicate that pure silver will form either rectifying contacts (nonohmic) or contacts with poor conductivity on gallium arsenide materials unless the carrier concentration is equal to or higher than 1.times.10.sup.18 carriers/cm.sup.-3 for n-types and 6.times.10.sup.18 carriers/cm.sup.-3 for p-types. Hence, applicant's ability to obtain good ohmic contacts with pure silver on n-type gallium arsenide materials at one order of magnitude lower than that taught by the prior art was totally unexpected, as will be more fully discussed hereinafter.
To continue, some pure metal contact systems such as molybdenum and chromium are problematic because they are extremely difficult to deposit. Some pure metal contact systems such as titanium and platinum are generally just as expensive as the gold alloy contact systems. Some pure indium contacts frequently have very low current drops at thresholds, and consequently are very unstable with time. Moreover, with the latter contacts failure by metal migration from the anode occurs rapidly. Notably, some pure tin contacts on bulk n-type materials often fail under bias by metal migration from the anode in the same way as pure indium contacts fail.
Some articles containing information relating to the forming of ohmic contacts for gallium arsenide semiconductor materials include: R. P. Gupta and J. Freyer, Metallization systems for ohmic contacts to p- and n-type GaAs, Int. J. Electronics, Vol. 47 No. 5, 459-467, July 1979; K. L. Kohn and L. Wandinger, Variation of Contact Resistance of Metal-GaAs Contacts with Impurity Concentration and Its Device Implications, J. Electrochem. Soc., Solid State Science, Vol. 116, No. 4 507-508, April 1969; H. Matino and M. Tokunaga, Contact Resistances of Several Metals and Alloys to GaAs, J. Electrochem. Soc., Electrochemical Technology, Vol. 116, No. 5, 709-711, May 1969; J. Palau, E. Testemale, Al Ismail, and L. Lassabatere, Surface and contact properties of GaAs overlaid by silver, J. Vac. Sci. Technol., Vol. 21, (1), 6-13, May-June 1982; and B. Schwartz, editor, Ohmic Contacts to Semiconductors, The Electrochemical Society, Inc., 1969.
Additionally, some articles containing information relating to the annealing process used in forming ohmic contacts for gallium arsenide semiconductors include: C. Lindstrom and P. Tihanyai, Ohmic Contacts to GaAs Lasers Using Ion-Beam Technology, IEEE Transaction on Electron Devices, Vol. ED-30, No. 1, 39-44, January 1983; B. L. Sharma, Ohmic Contacts to III-V Compound Semiconductors, Semiconductors and Semimetals, Vol. 15 1-39, 1981; and J. G. Werthen and D. R. Scifres, Ohmic contacts to n-GaAs using low-temperature anneal, J. Appl. Phys. 52(2), 1127-1129, February 1981.